gcc: don't build libssp on NetBSD

On NetBSD, this is provided by libc, and the GCC version clashes with it.
Disabling it matches the behaviour of pkgsrc on NetBSD.

Fixes: https://github.com/NixOS/nixpkgs/issues/119839

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -176,6 +176,7 @@ let
 
     # Platform-specific flags
     ++ lib.optional (targetPlatform == hostPlatform && targetPlatform.isx86_32) "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}"
+    ++ lib.optional targetPlatform.isNetBSD "--disable-libssp" # Provided by libc.
     ++ lib.optionals hostPlatform.isSunOS [
       "--enable-long-long" "--enable-libssp" "--enable-threads=posix" "--disable-nls" "--enable-__cxa_atexit"
       # On Illumos/Solaris GNU as is preferred